The present invention relates to a coordinate input device for controlling the position of a cursor on a display when the operator rolls a ball with his or her fingers in a desired direction. More particularly, the present invention pertains to an anti-static electricity structure for preventing entry of the static electricity charged on the operator to a control circuit or the like.
One example of the above-described type of coordinate input devices is a trackball designed to move the cursor on a display to a desired coordinate or menu position when the operator rolls with his or her fingers the ball partly protruding from a housing.
FIG. 5 is a perspective view of a conventional coordinate input device of the above-described type. A housing 1 made of a synthetic resin has a semi-spherical accommodating portion 2 which is open at both upper and lower surfaces thereof. A ball 3, which may be a steel ball, is rotatably accommodated in the accommodating portion 2. In the housing 1, a pair of metal rotary shafts 4 are rotatably provided substantially perpendicular to each other in such a manner that they are in contact with the ball 3. A code plate 5 is fixed to one end of each of the rotary shafts 4. A retainer 7 is mounted on the upper end of the housing 1 by an adequate fixing means, e.g., by using the bayonet mounting method (mounting the retainer 7 using teeth which are engaged by pressing and rotating the retainer 7 in an upper opening of the housing 1). The retainer 7 is made of a synthetic resin, and has an open portion 6 at the center thereof. The upper end of the ball 3 protrudes outwardly from the open portion 6 so that the operator can hand operate it. A printed circuit board 8 has a through-hole 9. The housing 1 is placed on and fixed to the printed circuit board 8 with the lower end of the ball 3 located within the through-hole 8 so as to avoid contact of the ball 3 with the printed circuit board 8. A static electricity eliminating pattern 10 is formed on the printed circuit board 8. A ring portion 10a is formed at one end of the static electricity eliminating pattern 10 on both the front and rear surfaces of the printed circuit board 8 and the cross-section of the ring portion 10a in such a manner that it surrounds the through-hole 9. A static electricity eliminating terminal 11 is soldered to the other end of the static electricity eliminating pattern 10. On the printed circuit board 8 are also formed detecting devices 12 each having a light-emitting element and a light-receiving element disposed such that they sandwich the code plate 5, a circuit device 13 constituting the control circuit, a connector 14 and so on. Both of the detecting devices 12 are connected to the circuit device 13 by a conductive pattern which is not shown.
In the thus-arranged coordinate input device, when the operator rolls the ball 3 slightly protruding from the open portion 6 of the retainer 7 mounted on the upper end of the housing 1 with his or her fingers in a desired direction, the two rotary shafts 4 rotate in predetermined directions synchronously with the rotation of the ball 3. Hence, the light-receiving element in each of the detecting devices 12 receives the light from the light-emitting device to detect the amount of rotation of the corresponding code plate 5. The control circuit operates the detection signals obtained by the detecting devices 12, and inputs the resultant X and Y coordinates of the ball 3 to a display device which is not shown. Thus, the cursor on the display, which is not shown, can be moved in a desired direction according to the amount and direction of rotation of the manually operated ball 3. The coordinate input or menu selection is executed by stopping the rotation of the ball 3 after the cursor has been moved to a desired coordinate or menu position and then by turning on an input switch which is not shown.
When the static electricity charged on the operator is discharged to the ball 3 during the manual operation of the ball 3, it is transmitted along the surface of the ball 3 to the ring portion 10a, and then released to a grounding circuit, which is not shown, through the static electricity eliminating pattern 10 and the static electricity eliminating terminal 11.
Since the conventional coordinate input device of the above-described type is constructed such that the static electricity charged on the operator is released via the ball 3 and then the static electricity eliminating pattern 10, entry of the static electricity into the control circuit or a circuit of an external device connected to the control circuit can be prevented, thus preventing erroneous detection or breakage of the circuit to a certain extent. However, since a fine gap is formed between the ball 3 and the ring portion 10a of the static electricity eliminating pattern 10 in order to reduce the contact resistance between the ball 3 and the ring portion 10a, part of the static electricity discharged to the ball 3 may be discharged to the rotary shafts 4 located between the ball 3 and the static electricity eliminating pattern 10 before it reaches the pattern 10. The static electricity released to the rotary shafts 4 may enter the control circuit through the code plate 5 and the detection devices 12, making the anti-static electricity structure an insufficient one.